Obviously this is a 64bit machine, so the qdata has to be 64bit aligned, but the ref_counter is only 32 bit. This leaves us with 32 bit of unused space. And at the same time we do all sort of bad-ass slow atomic hacks to extract the low two bits of the qdata pointer.

We could easily use two of these bits instead of the qdata bits on 64bit machines, and avoid lots of unnecessary atomic complex handling of qdata. And we might be able to use these extra bits for other (non-mandatory) performance tricks.